Sizing your boiler is the most common mistake in renovation — and the most costly. An oversized boiler cycles too often, consumes 15-20% more and wears out twice as fast. Undersized, it does not heat properly on cold days. Here is how to do the calculation correctly.
Why the “100 W per m²” rule is obsolete
This rule was designed in the 1980s for poorly insulated buildings. Today, a passive house uses 15 W/m², a renovated low-energy house 50 W/m², an old uninsulated house 120-150 W/m².
Applying a single rule leads to 30% oversized boilers in 7 out of 10 cases. The result: short cycling, parasitic condensation in the flue, lifespan halved.
The simplified heat-balance method
The correct calculation is done in three steps: (1) heat loss through walls, (2) air renewal, (3) surcharge for domestic hot water if combined production.
Wall losses = Σ (Area × U × ΔT). For a typical Belgian renovation: ΔT = 25K (outside −7°C guaranteed BE, inside 18°C comfort).
Air renewal = Volume × 0.3 (single-flow ventilation) to 0.15 (dual-flow ventilation) × 0.34 × ΔT (in watts).
Which type: gas condensing, oil, or heat pump?
Gas condensing remains the best efficiency (105-109% on net calorific value) for a contained budget. Vaillant ecoTEC plus and Bosch Condens GC9000iW are our best-sellers at ALPHA&CO.
Air/water heat pump has been mandatory in new builds since 2023 in Wallonia. Average seasonal COP 4.0 for Daikin Altherma and Atlantic Alfea. Investment +50% vs gas, but ROI 5-7 years with regional grants.
Oil condensing only in areas not served by gas, and with planned outdoor storage. Fuel cost unfavourable since 2024.
- Old uninsulated house: gas condensing (heat pump too large therefore inefficient)
- Renovated low-energy or new house: air/water heat pump or hybrid
- Passive house: heat pump + low-temperature underfloor heating (COP 4.5+)
